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I am implementing RQAOA in Cirq. After running regular QAOA to find an optimal state a (This I have done successfully).

I need to calculate $\langle a|Z_iZ_j|a\rangle$ for all $i,j$ in MyGraph.edges().

How should I go about using state a found with the QAOA circuit, to calculate the expectation value of a different circuit with that state?

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  • $\begingroup$ Welcome! Could you add some relevant links/code upon which others can build up upon? $\endgroup$ Mar 15 at 11:31
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If you've already simulated the final state $|a\rangle$, something like the following should work:

qubits = cirq.LineQubit.range(nqubits)

# qubit order in the observables must match the qubit order in the circuit used to generate |a>
qubit_map = dict(zip(qubits, range(nqubits))) 

for (i, j) in MyGraph.edges():
  # make the Z_i*Z_j observable
  ZiZj = cirq.Z(qubits[i]) * cirq.Z(qubits[j]) 
  # compute desired expectation
  expectation_ZiZj = ZiZj.expectation_from_state_vector(a, qubit_map=qubit_map)

Also in the expression $\langle a | B | a \rangle$, $B$ is generally not a quantum circuit, it needs to be an "observable" (Hermitian operator).

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  • $\begingroup$ Great that is exactly what I needed, thanks @forky40!! $\endgroup$
    – GuusH
    Mar 16 at 8:54

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